Method of preventing the formation of black stripes in the heat-treatment of metal bodies



Jan. 3, 1967 K. LOECK ETAL METHOD OF PREVENTING THE FORMATION OF BLACK STRIPES IN THE HEAT-TREATMENT OF METAL BODIES 3 Sheets-Sheet 2 Filed Dec. 22, 1965 Hans Weineck IN VEN TORS.

Attorney Jan. 3, 1967 METHOD OF PREVENTING THE FORMATION OF BLACK STRIPES IN THE HEATTREATMENT OF METAL BODIES K. LOECK ETAL Karl Loeck Josef Mo/z Hans We in eck IN VEN TORS.

Attorney United States Pa'tent C) ice METHOD OF PREVENTING THE FORMATION OF BLACK STRIPES IN THE HEAT-TREATMENT F METAL BODIES Karl Loeck, Oberhausen, Josef Molz, Mulheim an der Ruhr-Menden, and Hans Weineck, Oberhausen, Germany, assignors to Huttenwerk Oberhausen A.G., Oberhausen, Rhineland, Germany, a corporation of Germany Filed Dec. 22, 1965, Ser. No. 515,646 4 Claims. (Cl. 148157) ABSTRACT OF THE DISCLOSURE This disclosure relates to the heat treatment of metal bodies to be rolled or otherwise subjected to working at an elevated temperature in a pusher-type furnace and involves the transport of the bodies upon a pair of heatconductive rails with thermally insulated flanks which permit the heating of the bodies by nonoxidizing heating gas which is directed against the bodies from above and below during the entire displacement of each of the bodies from the inlet to the outlet, the bodies emerging without any delay in an equalizing chamber and being free from black stripes characterizing nonuniform heating.

This application is a continuation-in-part of our co pending application Ser. No. 196,420, filed May 21, 1962, now US. Patent No. 3,245,672 granted April 12, 1966.

Our present invention relates to the heat treatment of metal bodies such as steel billets, blooms, ingots and the like adapted to be successively processed in a pusher-type furnace and, more particularly, to an improved method of heat-treating such bodies so that the black stripes hitherto. commonly formed thereon and demonstrative of nonuniform heating of the bodies can be avoided.

Heretofore, pusher-type furnaces for the heating of billets and blooms generally have been formed with a preheating zone, main-heating zone and an equalizer or soaking zone successively arranged along a guide path for the bodies to be heated. The equalizing zone was required to permit temperature equalization within the heated body which often had portions in contact with the guide surfaces whose temperature was substantially below that of the remainder of the body so that so-called black stripes were formed. In the temperature-equalization chamber, the heat of' the hotter part of the body gradually raised the temperature of the cooler parts. The preheating and main-heating zones of such furnaces generally were provided with water-cooled guide rails terminating at a refractory-brick table within the equalizing zone. The disadvantages of this arrangement will become obvious when it is noted that such tables, which may be several meters long, often become encrusted with scale from the heated bodies as the latter are displaced therealong. This encrustation raises the transport surface of the table so that frequent cleaning is often required.

In the aforementioned copending application, we describe a furnace or oven in which the equalization zone is omitted and which nevertheless permits the heat treatment of the bodies without the development of such black stripes or other discoloration indicative of nonuniform heating. While the theory under which these so-called black stripes are formed has not been fully developed, it is believed that the commonly provided cooled supports maintain the proximal regions of the body at a substantially lower temperature while the balance of each body is heated strongly in the main-heating regions of the furnace. As a consequence, the rail-contacting parts of the body became discolored with respect to the remainder of the body and were also incompletely subjected to the heat treatment. Thus, by maintaining the bodies for prolonged Patented Jan. 3, 1967 periods in an equalization zone, it was possible to bring these underheated regions to the temperature of the remainder of the body. Besides the important disadvantages of loss of time, added capital expense in providing the equalization chamber and the difficulties involved in pushing the bodies not only through the equalizing zone, the prior-art systems were forced to convey the bodies along generally flat beds or tables of refractory brick because of the black-striping phenomenon resulting from use of rails. The oven described in this copending application was capable of avoiding these disadvantages.

The object of the present invention is to further develop the principles expounded in this copending application and provide an improved method of heat-treating metal bodies such that the black-striping phenomenon will not occur and that there will be no need for equalization zones as hitherto considered essential.

The above and other objects which will become apparent hereinafter are attained, in accordance with the present invention, by a method which involves the steps of successively feeding the metal (usually steel) bodies through a pusher-type heating-treating oven wherein the bodies slide along laterally insulated heat-conductive refractory members carried by fluid-cooled rails from an inlet of the oven to the outlet thereof with the bodies being then withdrawn from the oven, and directing a streamof heating gas against said bodies from above and below during the entire displacement of each of the bodies from the inlet of the oven to its outlet.

According to a more specific feature of this invention, the heating gas stream includes at least one gas current formed above the bodies by combustion of a fuel above the transport path, the hot combustion gases and the heat of combustion beingdirected downwardly against the bodies, with the gas passing in a direction counter to the direction of displacement of the bodies. A further gas current advantageous produced by combustion below the bodies is directed counter to their movement along their undersides while still another gas current can pass in the direction of movement of these bodies over part of their transport path from below before being diverted at a location proximal to the outlet, along the upper parts of these bodies to flow counter-current thereto.

According to another aspect of this invention, the upper and lower compartments of the furnace are generally subdivided into a preheating zone disposed adjacent the inlet opening and a main heating zone disposed adjacent the outlet opening and extending substantially up to the latter. The support rails also extend to the outlet opening and may, in fact, project therethrough. Thus, the use of an equalizing table or hearth is completely avoided.

The above and other objects, features and advantages of the present invention will become more readily apparent from the following description, reference being made to the accompanying drawing in which:

FIG. 1 is a longitudinal cross-sectional view schematically illustrating a furnace according to the invention;

FIG. 2 is a cross-sectional view taken along the line IIII of FIG. 1, with omission of the metallic bodies to be heated;

FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 1;

FIG. 4 is an enlarged elevational view, partly broken away, of the support rails; and

FIG. 5 is a cross-sectional view taken generally along the line VV of FIG. 4.

The drawing shows a pusher-type furnace .15 comprising a main housing 16 surrounding a refractory structure 17 forming an elongated furnace chamber generally designated 18. Furnace chamber 18 is provided at one axial end with an inlet opening 19 and, longitudinally remote therefrom, an outlet opening 7. These openings are spanned by a plurality of transversely spaced guide and supporting rails 2 which also subdivide chamber 18 into upper and lower compartments. Partitions 20, 21 intermediate the openings further subdivide these compartments into preheating zones 3, 4 and main heating zones 3' 4", respectively. The preheating zones 3', 4 are adjacent the inlet opening while the main heating zones 3", 4" extend to the outlet opening without the intermediary of an equalizing zone of the type previously required.

The guide rails 2 comprise longitudinally extending central supporting members 13 which are tubular and of generally prismatic configuration (see FIGS. 4 and A source of cooling fluid schematically shown as an arrow 22 supplies the tubular members 13 with cooling fluid. The tubular members carry respective rows of blocks 5 of heat-resistant but non-insulating character. These blocks 5 are of inverted-Y transverse cross-section and have arms 5' embracing with peripheral clearance the respective support member 13; the arms 5' form a channel 23 receiving the member. The roof. 24 of this channel is formed with protuberances 25 of limited surface area in contact with the upper surface of member 13 for maintaining a gas cushion between the latter and the blocks 5.

The underside of members 13 carries channel struts 26 which secure refractory bricks 27 to this underside in order to insulate it. The lateral surfaces of members 13 are provided with outwardly extending formations 28 receivable in recesses 29 in the inner surface of arms 5 to limit relative longitudinal displacement of the blocks and the members. The flanks of arms 5' are provided with a plurality of longitudinally spaced pins 30 adapted to engage plates 31 of refractory, heat-insulating material.

The heat-conductive upwardly extending flanges 6 of the blocks 5 of each row are substantially coplanar and have beveled corners 6 facing the inlet opening to cam oncoming bodies 1 (e.g. steel blooms or billets) upward ly onto the flat carrying surface of each flange. The carrying surfaces 6" operate in a substantially horizontal plane. Advantageously, the flange has a height n ranging between substantially 150 and 400 mm, and a transverse width w ranging between substantially 50 and 150 mm. The flanges 6 are also provided with bores 32 to facilitate their handling when the entire rail 2 is removed upon rolls 33 along the supporting surfaces of the furnace to permit the ready withdrawal of the rails along with these blocks 6, which may then be lifted from their supporting member 13 and replaced if. necessary because of encrustation etc. The rails may also be supported within the furnace by watercooled thermally insulated tubes 34 via hollow pillars 35 through which water is also circulated.

Partition 21 is formed with a chamber 36 admitting the heating fluid (erg. hot gas or a burning gas mixture) through a plurality of transverse apertures or burners 8 into the lower compartment 4', 4" of the furnace chamber into contact with the outsides of the bodies 1. A portion of the hot gas streams countercurrent to the direction of displacement 37 of the bodies 1 as indicated by the arrows 11, 11" while another portion of the .gas stream flows in the direction of displacement (arrow 11). A plurality of burners 9, 9 in the upper compartment 3', 3 provide part of the heating fluid for the upper surfaces of bodies 1, another portion of the fluid arriving from the lower compartments via gaps between the bodies. The cooled gases are removed from the furnace chamber via upright ducts 38 communicating with the lower compartment 4' and leading to an exhaust-gas conduit 12. A pair of lateral ducts 39 also leading to this conduit removes the cooled gases from the upper compartment 3, 3". Doors are provided in the preheating zone 4 4 of the lower compartment for the removal of scale whic tends to accumulate on the floor 40 thereof.

Example A succession of steel rollingunill billets is passed through a pusher-type furnace at a rate selected with respect to the dimensions of the bodies such that, in the preheating zone 3, 4', the bodies are brought to temperatures in the region of. approximately 880 C. and thereafter are raised to the desired rolling temperature, generally about 1350" C. The bodies are examined after being withdrawn from the outlet and are found to be totally free of black stripes. When, however, the heateonductive refractory bodies 6 are disposed with all other conditions remaining the same, the bodies recovered at the outlet are found to have black stripes along their underside indicative of nonuniform heating. In this case, it is necessary to maintain the bodies in a soaking or equalizing zone at a temperature of about 1320" C. to restore homogeneity. When the refractory bodies 6 are used and the rails 13 are not water-cooled, rapid deterioration of the support structure is observed. Furthermore, when the contacts of the bodies with the gas stream is not carried out over the entire furnace length from inlet to outlet and is terminated in advance of the outlet at a distance therefrom corresponding substantially to the length of conventional soaking or equalizing zones, the heat utilization of the furnace is found to be poor and the bodies are found to be of nonuniform temperature.

Thus, this method permits the heating of billets, blooms, ingots and the like without the formation of black stripes indicative of cool regions along those portions of the bodies contacted by the support rails, since the flanges 6, directly in contact with the bodies, are substantially at the furnace temperature and are insulated from the watercooled support member by the air cushions. The rails 2 thus can extend to or through the outlet opening 7 inasmuch as no equalizing heart-h is required. Consequently, a saving in material and labor as well as increased heating efliciency is obtained.

The invention described and illustrated is believed to admit of many modifications within the ability of persons skilled in the art, all such modifications being deemed included within the spirit and scope of the appended claims.

We claim:

1. In the heat treatment of metal bodies which are successively fed through a pusher-type heat-treating oven from an inlet of said oven to an outlet thereof by sliding said bodies along fluid-cooled rails having laterally insulated heat-conducting refractory members interposed between said bodies and said rails, the bodies being thereupon withdrawn from said oven, the improvement which comprises the step of directing a stream of heating gas against said bodies from above and below ur-ing the entire displacement of each of the bodies from said inlet to said outlet.

2. The improvement defined in claim 1 which further comprises the step of forming at least part of said gas stream by combustion of a fuel in at least one chamber above said bodies whereby the combustion gases and the heat thereof are directed against said bodies in a direction counter to the movement of said bodies through said oven, said stream including a current of hot gases formed by combustion of a fuel below said bodies contacting said bodies from below while passing in a direction counter to the movement of said bodies through said oven.

3. The improvement defined in claim 2 wherein at least part of the gas stream contacting said bodies from above is formed by a gas current originally flowing in the direct-ion of displacement of said bodies and therebelow, the last-mentioned current passing above said 5 6 bodies proximal to said outlet, said improvement fur- References Cited by the Examiner ther oomprising the step of drawing said gas inorn said UNITED STATES PATENTS oven proximal to said inlet.

4. The improvement defined in claim 3, further com- 4/1920 Egler prising the step of subdividing the path of said bodies 5 FOREIGN PATENTS through said :ove-n only into a preheating zone proximal to said inlet and a main heating zone extending c-om- 1,094,778 12/1960 Genmany' pletely from said preheating zone to said outlet forming by oombustion of a duel hot gas streams in each of said DAVID RECK Pnmary Exammer' zones above and below the bodies. 10 C. N. LOVELL, Assistant Examiner. 

1. IN THE HEAT TREATMENT OF METAL BODIES WHICH ARE SUCCESSIVELY FED THROUGH A PUSHER-TYPE HEAT-TREATING OVEN FROM AN INLET OF SAID OVEN TO AN OUTLET THEREOF BY SLIDING SAID BODIES ALONG FLUID-COOLED RAILS HAVING LATERALLY INSULATED HEAT-CONDUCTING REFRACTORY MEMBERS INTERPOSED BETWEEN SAID BODIES AND SAID RAILS, THE BODIES BEING THEREUPON WITHDRAWN FROM SAID OVEN, THE IMPROVEMENT WHICH COMPRISES THE STEP OF DIRECTING A STREAM OF HEATIN GAS AGAINST SAID BODIES FROM ABOVE AND BELOW DURING THE 